1. Field of the Invention
The present invention relates to a method and an apparatus for supplying a glass forming raw material. More particularly, the present invention relates to a method and an apparatus for supplying a desired amount of a glass forming raw material into a reactor with good reproducibility. Typically, the glass forming raw material is one used for producing a glass preform from which an optical fiber is fabricated.
2 Description of the Related Art
The typical methods for producing a glass preform for use in the fabrication of an optical fiber include the inside vapor deposition method, the outside vapor deposition method and the vapor phase axial deposition method. In these methods, the glass forming raw material such as silicon tetrachloride (SiCl.sub.4) and an additive for modifying a refractive index of the formed glass (e.g. germanium tetrachloride, phosphorus oxychloride and boron tribromide) are filled in raw material containers, respectively and then a carrier gas such as argon and helium is bubbled in the raw materials with controlling the flow rate of the carrier gas to evaporate the raw materials. After adjusting the saturation ratios of the raw material vapor, the mixture of the raw materials and the carrier gas is supplied into the reactor.
FIG. 1 schematically shows a conventional apparatus for supplying the glass forming raw material in a reactor. In this apparatus, the carrier gas is bubbled into the glass forming raw material 3 in a container 2 through a flow controller 1 for adjusting the flow rate of the carrier gas, and the evaporated gas is supplied in a reactor 4.
The flow rate m of the glass forming raw material can be calculated according to the following equation: ##EQU1## wherein m is a flow rate of the glass forming raw material, C is a saturation coefficient, M is a flow rate of the carrier gas, T is a temperature of the container, P.sub.T is a vapor pressure of the glass forming raw material at the temperature T, and P.sub.0 is a pressure of an atmosphere surrounding the container for the raw material and the reactor.
To keep the flow rate m of the glass forming raw material constant, all the variables in the right side of the equation (1) should be kept constant.
A technique for keeping the saturation coefficient C constant is disclosed in Japanese Patent Publication No. 1378/1986. According to this technique, two containers for the glass forming raw material are connected in series with a pipe for supplying the carrier gas, and the temperature of the first container is kept higher than that of the second container which is closer to the reactor. Such arrangement of the containers keeps the saturation coefficient C constant at about one (1) with good reproducibility. In addition, the temperature T of the container and the carrier gas flow rate M can be kept constant with a temperature regulator and a mass flow controller, respectively with good reproducibility.
However, according to the conventional method for supplying the glass forming raw material, the flow rate of the glass forming raw material changes delicately even if the saturation coefficient C, the carrier gas flow rate M and the container temperature T are kept constant. Thus, the flow rate of the glass forming raw material cannot be kept constant with good reproducibility.